Image guided radiation therapy (IGRT) uses images, such as treatment planning images obtained prior to radiation delivery or intra-treatment images obtain during treatment delivery, to identify the location of a treatment target (either directly or relative to a known structure within the body of a patient) within a treatment room reference frame relative to a treatment planning image reference frame. In IGRT, challenges arise when attempting to locate a target region (or a structure) within the body of the patient that moves, either just prior to, or during the course of radiation treatment, from its location within the body when the treatment planning image was acquired. Image registration provides the ability to locate a target region within the body by comparing the image content between two or more images.